TiCl4/MgCl2-alkoxysilane-based catalysts for olefin polymerisation were prepared by supporting TiCl4 on recrystallised M.-Cl, containing grafted alkoxysilane. The alkoxysilanes were generated in situ during the recrystallisation of MgCl2 with SiCl4 from alcoholic solutions. The effect of the nature of the alkoxysilane on the catalytic performance and polymer properties was studied by using different alcohols as solvent. The results of the polymerisation of ethylene and propylene showed that the activities, molecular weights and isotacticities depended strongly on the structure of the alkoxysilane, which possibly acts as an internal electron donor. (c) 2004 Elsevier B.V. All rights reserved.